1. Field of the Invention
This invention relates to an optical system and an optical scanning apparatus having the optical system used for devices such as laser beam printers.
2. Description of the Related Art
Many rotary polygons are used for optical systems of devices such as laser beam printers in order to deflect a laser beam at a high speed. The beam deflected by the rotary polygon forms a beam spot on a target surface (for example, a surface of a photo conductor drum) and the beam spot travels in the fixed direction with the rotation of the rotary polygon to form a scanning line.
When a tilt error or a field tilt due to a manufacturing error or an installation error exists in a reflective surface of the rotary polygon, the location of the beam spot on the target surface will shift and unevenness will arise between the scanning lines. In order to prevent the unevenness, a field tilt correcting optical system is typically applied to make a substantial conjugate relation between the reflective surface of the rotary polygon and the target surface. For example, Japanese Patent Laid-open No. 6-230307 discloses locating a cylindrical lens between the rotary polygon and the target surface. Japanese Patent Laid-open Nos. 58-132719 and 8-278464 disclose locating a toroidal lens between the rotary polygon and the target surface.
However, the longer the distance between the cylindrical lens and the target surface is, the larger the curvature of field in a sub-scan direction becomes, in using only a cylindrical lens as a field tilt correcting optical system. Therefore, the cylindrical lens should be located near the target surface. Moreover, the cylindrical lens should be manufactured to have longer length, which increases the manufacturing cost of the optical system, because the cylindrical lens requires a high processing cost. Especially, as high resolution is required, the tolerance of the curvature of field becomes smaller and the problem becomes more remarkable. Moreover, in using the toroidal lens as a field tilt correcting optical system, there is a problem that the manufacturing cost further increases as the scanning stroke becomes longer, that is, a longer toroidal lens is needed, because the toroidal lens itself requires a high processing cost.
Especially, in recent years, about 600 mm scanning stroke has been needed for devices such as laser beam printers, in order to form big size images (for example, A1 size). Furthermore, the resolution of about 600 dpi has been required for improvement in image quality. Thus, as the scanning stroke becomes longer and the resolution becomes higher, the problem of the manufacturing cost becomes more remarkable in both field tilt correcting optical systems described above.
Then, Japanese Patent Publication No. 4-21164 discloses a field tilt correcting optical system using a cylindrical lens and a cylindrical mirror. In the field tilt correcting optical system, the cylindrical lens is positioned to the side near the rotary polygon, and the cylindrical mirror is positioned to the side near the target surface. Such an arrangement allows a comparatively longer cylindrical mirror and a comparatively shorter cylindrical lens. The processing cost of cylindrical mirrors is cheap compared with cylindrical lenses, and such configuration contributes to reduction of the manufacturing cost of the optical system.
However, in such a field tilt correcting optical system, since a coma aberration occurs due to the reflection of the beam on the cylindrical mirror, a spot with the secondary intensity called a side lobe is formed adjoining the original beam spot on the target surface. Such a side lobe prevents diameter reduction of the beam spot and it is difficult to improve the resolution of images, which is a problem.
The present invention has been achieved in view of the above problems. It is an object of the invention to provide an optical system for scanning to form a minute beam spot and also to be applicable to the formation of big size images.
The present invention provides an optical system for scanning a predetermined target surface in a first scanning direction, comprising: a light source emitting a beam; a first image forming optical system for focusing the beam emitted from the light source in a direction perpendicular to a direction corresponding to the first scanning direction, thereby forming an image; deflecting means for deflecting a beam come out from the first image forming optical system so that the beam scans the target surface in the first scanning direction; a second image forming optical system for focusing the beam deflected by the deflecting means on the target surface and thereby forming an image so that a scanning speed on the target surface is uniform all over a scanning stroke; a cylindrical lens, located in an optical path between the second image forming optical system and the target surface, having negative refractive power only in a second scanning direction perpendicular to the first scanning direction; and a cylindrical mirror, located in an optical path between the cylindrical lens and the target surface, having refractive power only in the second scanning direction; wherein the scanning stroke in the first scanning direction on the target surface is longer than 500 mm; an incident angle of a paraxial beam passing through the second image forming optical system and entering the cylindrical mirror is 15 degrees or less in the second scanning direction; and a condition shown in expression (1) as follows is satisfied:
0.03 less than DX/DL less than 0.06xe2x80x83xe2x80x83(1) 
in the expression, DX is a beam size in the second scanning direction of the beam entering the cylindrical mirror and DL is a distance between the cylindrical mirror and the target surface.
In the optical system for scanning of the invention, the generation of the coma aberration in the second scanning direction can be suppressed, and therefore the generation of the side lobe on the target surface can also be suppressed because the incident angle of the beam entering the cylindrical mirror is 15 degrees or less in the second scanning direction. In addition, the formation of big size images may be achieved because the scanning stroke in the first scanning direction is longer than 500 mm. It should be noticed that the size in the second scanning direction of the beam spot formed on the target surface becomes smaller, as the beam size DX in the second scanning direction of the incident light into the cylindrical mirror is greater. And the required length of the cylindrical mirror becomes shorter, as the distance DL between the cylindrical mirror and the target surface is longer. In addition, with the optical system for scanning of the invention, since expression (1) is satisfied, the sufficient minute beam spot to obtain the expected resolution in the second scanning direction can be acquired, and the length of the cylindrical mirror can be shorter to an extent that manufacturing can be performed without difficulty.
The present invention provides an optical system for scanning a predetermined target surface in a first scanning direction, comprising: a light source emitting a beam; a first image forming optical system for focusing the beam emitted from the light source in a direction perpendicular to a direction corresponding to the first scanning direction, thereby forming an image; deflecting means for deflecting a beam come out from the first image forming optical system so that the beam scans the target surface in the first scanning direction; a second image forming optical system for focusing the beam deflected by the deflecting means on the target surface and thereby forming an image so that a scanning speed on the target surface is uniform all over a scanning stroke; a cylindrical lens, located in an optical path between the second image forming optical system and the target surface, having negative refractive power only in a second scanning direction perpendicular to the first scanning direction; a cylindrical mirror, located in an optical path between the cylindrical lens and the target surface, having refractive power only in the second scanning direction; wherein the second image forming optical system comprises a first lens component having negative refractive power in the first scanning direction, a second lens component having positive refractive power in the first scanning direction, and a third lens component having positive refractive power in the first scanning direction in order from a side close to the deflecting means, and conditions shown in expressions (2) to (5) as follows are satisfied.
F/DY less than 25xe2x80x83xe2x80x83(2) 
0.2 less than xe2x88x92F1/F less than 0.35xe2x80x83xe2x80x83(3) 
0.6 less than F2/F less than 0.9xe2x80x83xe2x80x83(4) 
0.3 less than F3/F less than 0.7xe2x80x83xe2x80x83(5) 
in the expressions, F is a focal length of the second image forming optical system;
DY is a beam size in the first scanning direction of the beam entering to the second image forming optical system;
F1 is a focal length of the first lens component;
F2 is a focal length of the second lens component; and
F3 is a focal length of the third lens component.
In the optical system for scanning of the invention, the size in the first scanning direction of the beam spot formed on the target surface can become shorter because the expression (2) is satisfied. Moreover, since the expressions (3)-(5) are satisfied, generation of spherical aberrations and coma aberrations can be prevented, and the curvature of field and distortion aberration in the first scanning direction are corrected properly.
Other and further objects, features and advantages of the invention will appear more fully from the following description.